Quantification of seasonal variation in herd performance contributes to management planning and decision-making. Our objectives were to study the association between measures of seasonality of milk yield (MILK), somatic cell count (SCC), and herd size (SIZE) across the US in 2016. The regions analyzed (herds) were Southeast (SE, 183), Northeast (NE, 2,332), and Midwest (MW, 2,240). We used linear regression models with sine and cosine functions to describe seasonality per herd per lactation (LACT) to find the annual low (minimum) and peak (maximum) for each variable. Annual low-to-peak ratios were measures of maximum seasonality, and they were not adjusted (LP) or adjusted (LPADJ) for days in milk to correct for seasonal calving. Traditional summer-to-winter (SWr) ratios of unadjusted observations were used for comparison. We excluded cows with fewer than 5 test-day observations, herds with fewer than 15 cows per LACT group, and farms that were expanding or shrinking. Overall, the correlation between SWr and LP were 26% for MILK, but −26% SCC. The correlation between seasonality in MILK and SCC was significant but not high across herds (10% for both LP and LPADJ). The correlations between SIZE and seasonality of MILK were 13% (LP) and 12% (LPADJ), and 25% for SCC (for both LP and LPADJ). An increase of 100 cows decreases seasonality (LP) in MILK by 0.003 in all regions, a percentage of 2.9% (SE), 1.8% (NE), and 1.9% (MW) and in SCC by 0.008 (0.6%, SE), 0.015 (2.7%, NE), and 0.014 (2.5%, MW). The difference between LP and LPADJ decreased by 0.0006 (0.3%) for every increase in 100 cows in herd size for MILK, and by 0.0001 (6%) for SCC. In conclusion, LP were better measures of seasonality in MILK and SCC than SWr. Large farms were less seasonal than small farms for both MILK and SCC in all regions. Small farms had the greatest reduction in seasonality when adjusted for calving pattern.Our work highlights the importance of using appropriate measures to avoid under or overestimation of the true seasonality. Calving pattern should be taken into consideration by the industry when implementing seasonal milking price and milk quality plans.